The present invention relates to a method and apparatus for limiting the propagation of ductile fractures in pipelines used to transport fluids such as natural gases, compressed gas liquids or liquid-natural gas mixtures under pressure (hereinafter "natural gas fluids"). More particularly, it relates to crack arrestors that may be put in place during fabrication of a pipeline or added to existing pipelines without degrading their quality. The system is particularly useful for long distance pipelines such as contemplated for the transport of natural gases and hydrocarbons from the Arctic.
It is known that it is very costly to fabricate and assemble highly crack resistant large diameter pipe suitable for use as pressurized natural gas transmission lines. In any event, it is not practical to design a pipeline that will under all conditions withstand the internal stresses or external shocks that may cause cracks in a pipeline. In addition, it is known that a crack of substantial size in such pipeline will rapidly enlarge such that its ends may propagate along the pipeline for extremely long distances. These propagating cracks are of two types: brittle and ductile. Brittle cracks propagate at high velocities, typically, 1,300-3,000 ft./sec. without much apparent deformation of the pipeline near the crack. The pipeline appears to break open and thereby relieve the stress causing the original crack. Ductile fractures propagate at lower velocities, typically, 250-1,000 ft./sec. and are associated with substantial pipeline deformation. Ductile fractures can propagate for substantial distances, up to several miles, ripping open the pipeline as if it was unzipped. Typically, ductile fractures run axially along the pipeline. Ductile fractures predominate above a critical temperature termed the brittle-ductile transition temperature, although, depending upon the means of fabrication, brittle fractures may be found at elevated temperatures.
There is a belief that the propagation of ductile fractures is made possible by the pressure of the natural gas in the pipeline against the flaps forming the edges of the crack. By that mechanism the crack may be driven forward at a velocity equal to the velocity of the low pressure front caused by the escaping gas. In this manner, although the gas pressure in the pipeline may rapidly drop as a result of gas escaping through the crack, there is sufficient pressure at the flap to cause the crack to propagate until it is arrested in some manner.
Many devices and methods have been discussed to arrest the propagation of ductile fractures. Some suggest attaching large masses to the pipeline to cause the fracture to deviate from its straight line path into a helical path in the hope that that will permit the low pressure front to catch up with the crack and result in an arrest. Another method that has been suggested in the copending application of R. Eiber, Ser. No. 665,547 and Loncaric U.S. Pat. No. 3,870,350 is to periodically interpose along the pipeline more brittle sections. This is believed to cause arrest at the end of the brittle section where the fracture again becomes ductile, due to the absence of the flap driving force. These latter arrestors would seem to have the disadvantage of artificially lowering the crack resistance of the pipeline: crack initiation becomes more probable although less destructive.
Crack arrestors have been suggested to interfere mechanically with the dynamic mechanism believed to sustain crack propagation. Thus, flexible sleeves or hoops which are an integral part of the pipe have been suggested to restrain flap formation. See "Bulletin" Research Laboratory, U.S. Steel Corporation, Apr. 10, 1974. Such flexible sleeves or hoops have been suggested to encircle the pipeline, in contact, and at zero or greater tension. Typically, it is suggested that such arrestors should be made of the same material as the pipeline. Crack arrestors have been suggested to interfere with the thinning of the pipeline in the region of the propagating crack tip. These arrestors have been designed to tightly restrain the circumference of the pipeline. See U.S. Pat. Nos. 2,401,092 (wire wound at high tension), 3,349,807 (steel strap or band at high tension), and 3,631,897 (tensioning strands).
Each of the above-mentioned crack arrestors suffers from the defect of requiring intimate contact as an integral part of the pipeline. See also Risley U.S. Pat. No. 3,096,105 (arrestor welded to pipe section). This defect is enhanced by the fact that the pipeline itself undergoes circumferential expansion as a result of the internal pressure of the transported gas. Thus, a balance must be maintained between the size of the arrestor and the pipeline's response to being pressurized. A sleeve type crack arrestor that is tight when the pipeline is brought up to operating pressure will introduce bending stresses into the pipe at the edge of the arrestor that could contribute to the formation of a crack in the pipe which is detrimental.
Plain sleeve type members have also been suggested in connection with laying deep underwater pipelines. See, e.g., Ells U.S. Pat. No. 3,860,039. There outside pressure on the pipeline predominates over internal pressures during the laying operation and the major problem is to restrain buckling associated with pipe collapse and not crack propagation. For that purpose it has also been suggested to weld reinforced sections into the pipeline.